The present invention relates in general to electrical modules providing protection to telephone lines, and more particularly to an economical 5-pin protector that facilitates the manufacture and assembly thereof.
Telephone line protection devices have been required for many years by telephone operating companies, and the like, in order to provide protection to central office switching equipment from electrical strikes and power line crosses coupled to the telephone lines. At least one telephone line protector is required for each telephone subscriber line. In view that many central office switching systems service thousands, and often hundreds of thousands of subscribers, many such protection modules are required. Each module is constructed of a standardized size and with five pins of a uniform configuration. Two pins are associated with the tip and ring telephone lines, two other pins are associated with the customer or equipment tip and ring lines, and a fifth pin is connected to ground.
The basic function of a telephone line surge protection circuit is to sense an excessive voltage, such as 300 volts, on either the telephone line tip or ring conductor, and very quickly connect such conductor to ground to prevent the customer equipment from experiencing sustained voltages higher than such threshold. Specialized solid state devices are generally utilized for providing a high speed short circuit to ground. A host of other subsidiary features can be carried out by the protection device, including fail-safe grounding in response to a thermal overload, open circuiting between the telephone line and customer lines, etc.
Because of the high demand for protection devices, a myriad of such devices are currently available. Illustrative of the many different protection device designs and modules are those set forth in U.S. Pat. No. 4,796,150 by Dickey et al.; U.S. Pat. No. 5,031,067 by Kidd et al; U.S. Pat. No. 5,172,296 by Kaczmarek; and U.S. Pat. No. 5,357,568 by Pelegris. In all of these protection module designs, there are many complicated parts which lead not only to greater cost, but also to reduced reliability. Importantly, because of the complicated nature of the components of the various modules, the manual labor in assembling the modules is extensive, which only leads to a more costly product.
It is well-known in the electronics industry to facilitate assembly of modules by mounting the components on printed circuit boards. Indeed, the printed circuit board manufacture as well as the automatic component insertion is well-established. The use of printed circuit boards in protection devices is suggested in U.S. Pat. No. 5,175,662 by DeBalko et al. Various semiconductor devices and components are mounted on a printed circuit board which, in turn, is connected by connector contacts to a module base. Although this design tends to be more efficient, it still requires the additional expense of connectorizing the printed circuit board as well as requires a molded plastic base for the module. The injection molding of the base members of many of the protection modules is a standard practice, as noted in U.S. Pat. No. 5,357,568 by Pelegris. Disclosed in U.S. Pat. No. 5,442,519, also by DeBalko et al, is a pair of printed circuit boards that are utilized in a telephone line protection module. However, one printed circuit board is still connected to a plastic base member by way of pin and socket connectors, thereby resulting in a relatively expensive unit. The use of plastic base members is popular, primarily because of the ease of forming tabs at the peripheral edges thereof for snap-lock engagement with corresponding holes in a protective cover. Such an arrangement is shown in the Dickey et al. patent identified above.
In many of the conventional telephone line protection modules, encapsulated semiconductor devices are utilized as overvoltage sensors and switches. The reason for this is that such devices are generally available from the manufacturer only in packaged or encapsulated form. Again, the additional processing of the devices to package the same results in a more expensive component, often when packaged devices are not necessary. For example, when a semiconductor device is packaged so as to have pins, then often a corresponding socket is required. It can be appreciated that the use of packaged components often leads to a more expensive product.
From the foregoing, it can be seen that a need exists for a new type of protection module that has few components, is easily assembled and is much less costly than the prior art modules. Another need exists for an improved telephone line protection module that employs a printed circuit board as the base itself to which the module pins are inserted, thereby facilitating both manufacture, assembly and cost. Another need exists for a protection module design in which the protective cover can be snap-lock engaged directly to a printed circuit board base member.
In accordance with the principles and concepts of the invention, a protection module is disclosed of the type that overcomes the shortcomings and problems attendant with the prior art designs. In accordance with the preferred embodiment of the invention, the telephone line protection module includes a printed circuit board base member to which five module pins are mounted. The printed circuit board is formed with three general conductive areas, two for connecting the respective telephone line tip and ring conductors to the corresponding tip and ring customer equipment, and a third conductive path providing a ground connection to the module circuits.
First and second unpackaged semiconductor wafer devices that are responsive to overvoltage conditions are soldered to a conductive bridge member. The center of the bridge member is then soldered to the printed circuit board ground connection, while the ends of the bridge member cause the semiconductor wafers to be forced into electrical contact with respective printed circuit board tip and ring conductive paths. A generally C-shaped spring with curved ends is captured within web structures of a housing cover so that when the cover is snap-fit to the printed circuit board base, the spring wire bears down on the ends of the conductive bridge, thereby forcing the semiconductor wafer devices into contact with the respective tip and ring conductive paths. A fail-safe mechanism is provided, in that if either one of the semiconductor wafers is thermally destroyed due to excessive current therethrough, the spring tension causes the conductive bridge to be forced into contact directly with the conductive path of the printed circuit board. A telephone line protection module of very few components, and which is easily assembled, is thereby provided.
In accordance with a second embodiment of the invention, the module case does not snap-lock to the printed circuit board base, but rather has a housing cover with a plastic post that protrudes through the base. The end of the post can thus be melted and mushroomed to thereby secure the base to the module cover.